There are four basic types of frequency synthesizers used in conventional frequency hopping communication systems. These include indirect synthesizers, direct synthesizers, hybrid combinations of indirect and direct synthesizers, and phase modulating synthesizers.
An indirect synthesizer includes a programmable counter for dividing the output frequency of a voltage controlled oscillator (VCO) by N, and a phase detector for comparing the output frequency of the counter with that of a reference frequency. The detector generates an error voltage proportional to the difference between these two frequencies which drives a phase-lock loop that maintains the output frequency of the VCO at N times that of the reference. The output frequency of the VCO can therefore be changed by programming the variable counter to any desired ratio N.
The switching speed of this type of synthesizer depends on the gain bandwidth characteristics of the phase-lock loop and the propagation delay through the variable counter. However, the switching speed of an indirect synthesizer is usually slow and is generally in the millisecond range due to the settling time in the phase-lock loop circuits. Though indirect synthesizers are simple to build, they are not suitable for high hopping rate synthesizer applications. A ping-pong technique, however, can be used to alleviate the problem associated with low switching speed. In particular, while one synthesizer provides the operating frequency, the second is being set to the next new frequency. The second unit is switched in at the time the new frequency is required. This process then repeats with the first unit in a ping-pong fashion.
A direct synthesizer consists of comb generators, frequency multipliers, filters, and switches arranged in mix-and-divide-add-and-divide circuits, or a combination of both. In this type of synthesizer, all of the required carrier frequencies are always present. A narrow band filter filters out unwanted frequency components from each desired carrier, while banks of switches are used to select the desired frequency. The switching speed of a direct synthesizer depends on the switching speed of the switches, the propagation delay of all the cascaded circuits between the source and the final output, and the bandwidth of the narrowest filter in the cascaded circuits. The delay due to filter bandwidth is 0.5/BW, where BW is the filter's 3 dB bandwidth.
Hybrid combinations of direct and indirect synthesizers replace some or all of the switchable rf sources with indirect phase-lock loop synthesizers. Since each of these phase-lock synthesizers can generate a much larger number of base frequencies than either frequency multipliers or comb generators, the number of multipliers, mixers and switches required for a given number of frequencies can be greatly reduced.
A phase modulating synthesizer causes a frequency shift in the carrier by changing the accumulated incremental phase shift as a function of time. A conventional technique is to use a tapped delay line with M taps, thereby dividing the period of the carrier frequency into M equal time steps. When the M delay taps are sequentially selected in equal discrete time steps of T.sub.n /M, for example, the carrier frequency will be shifted by f.sub.n =1/T.sub.n.
Another example of a phase modulating synthesizer is to use a quadrature phase-shift-keyed (QPSK) modulator. A QPSK modulator consists of two BPSK (bi-phase-shift-keyed) modulators connected in quadrature. Each BPSK modulator consists of hybrids and Schottky barrier diodes. Since Schottky barrier diodes are two-state on-off devices, four discrete phase states of -45.degree., -135.degree., -225.degree., and -315.degree. corresponding to the four data values 11, 10, 00, and 01 are obtained. When these four phase states are switched cyclically in equal time intervals, a frequency shift will result. The frequency shift is given by the reciprocal of the total time it takes to cycle through the four data values.
The performance of a frequency shifter is evaluated by examining the levels of feed-through carrier and the undesired sidebands. The level of feed-through carrier depends on how the modulator is configured. In a QPSK modulator, the carrier feed-through depends on the phase and amplitude balance between the circuits. In a discrete phase accumulative type of frequency shifter, both the frequency spacing and the level of undesired side bands depend on M, the number of phase steps used. If f.sub.o is the applied carrier frequency and f.sub.m is the applied frequency shift, then the desired new carrier frequency is f.sub.o -f.sub.m (or f.sub.o +f.sub.m when the accumulated phase increases with time). The side bands are spaced at Mf.sub.m apart. The upper side band level is -20Log(M-1), and the lower side band is -20Log(M+1). (The reverse is true for the f.sub.o +f.sub.m case.) For M=4, the two side band levels are -9.5 dB and -14.1 dB. For M=8, these side band levels are -16.9 dB and -19.1 dB.
From the above, it can be seen that for phase accumulative frequency shifters, levels of undesired side bands can be reduced only by increasing the number of phase accumulation steps. This will result in increased complexity of the synthesizer due to an increased number of phase accumulation stages. Increasing the number of time steps not only slows down the switching time between frequencies, but also imposes an upper frequency limit in f.sub.m.